Dewaxing treatment of petroleum fuel oils



United States Patent F This invention relates to the treatment of petroleum I fuel oils for the purposes of improving their low temperature properties, particularly their cloud points. The invention is more particularly concerned with light residual fuel oils which are intended to be handled in unheated storage, e.g. those meeting British Standard Specification No. 2,869; 1957, Class E. Residual fuel oils are those containing residues from the distillation of crude petroleum.

When a fuel oil is cooled sufliciently, there comes a point when wax starts to separate from the oil. This point is known as the cloud point of the oil. Further cooling of the oil below its cloud point causes increasing quantities of wax to separate from the oil until eventually the oil ceases to flow. This point is known as the pour point. Methods of determining the cloud point and pour point of petroleum oils are described in the British Institute of Petroleums Standard Method No. 15.

r Separation of wax from a fuel oil can cause blocking of filters and have other undesirable effects and recently various additives have been proposed for lowering the cloud point of fuel oils. In general, however, these have not produced a sutficient lowering of cloud point to render their use attractive.

A process has now been developed for treating petroleum fuel oils which generally has the effect of lowering the cloud point of the oil by a substantial amount, and of improving the filterability of the oil at low temperatures, without substantially altering the composition of the oil. 1

According to the invention, there is provided a process for treating a petroleum fuel oil having a viscosity within the range 5-100 centistokes (40-400 seconds Redwood l) at 38 C. (100 F.) and a specific gravity at 16 C. (60? F.) in the range 0.90-0.96, which comprises separating from the fuel oil by means of a solvent, substantially all of the wax consisting of wax molecules having carbon numbers of 35 or more, whilst leaving in the oil substantially all of the wax consisting of wax molecules having carbon numbers of less than 30. In most cases this separated wax'represents of the order of 5-25%, usually 515%, by weight of the total wax originally present in the fuel oil. I

Any dewaxing solvent maybe used. A particularly satisfactory solvent is methylenechloride. Other suitable solvents include acetone, methyl ethyl ketone or mixtures of these solvents with an aromatic solvent such as benzene or toluene.

The solvent treatment may conveniently be carried out by mixing the fuel oil and excess solvent together at a temperature at which all the constituents of the oil except the wax to be separated go into solution, filtering off the undissolved wax and removing the solvent from the filtrate. With most light fuel oils having the characteristics specified it will be possible to separate the necessary quantity of wax by carrying out the solvent treatment at normal ambient temperatures e.g. 227 C. The amount of wax separated depends on the temperature of treatment, lower temperatures resulting in more wax being separated and vice versa. The necessary tem- F S0 extract of kerosine 3,249,527 Patented May 3, 1966 perature required to separate the desired amount of Wax in any particular case can easily be determined by simple experiment. The process is particularly applicable to residual fuel oils. These are usually prepared by blending distillation residues with lighter distillate fractions such as gas oil or kerosine and also sometimes with materials discarded during the production lubricating oils and residual materials from cracking processes. The blending is carried out at elevated temperature, usually of the order of 4960.C. The process of the invention may be applied to fuel oils which have been cooled from the blending temperature to the solvent treatment temperature in the conventional manner i.e. slowly (e.g. about 1 C./day). Filtration of the separated wax is easy when the fuel oils have been cooled in this manner presumably because the particle sizes of the wax crystals are large. The process may also, however, be applied with advantage to fuel oils which have been cooled by the ripid cooling technique described in UK. patent specification 851,135.

The process may also be applied to 'fuel oils which have been subjected to a conventional deasphalting treatment, e.g. with propane or boiling acetone, or those prepared Without the inclusion of distillation residues. v

By way of example, four fuel oils A, B, C and D were treated by the process of the invention. The properties of the fuel oils are given in the table below: their compositions were as follows:

Percent wt. Vacuum residue 64.5 Gas oil 28.0 S0 extract of kerosine 7.5

Fuel oil A which had been deasphalted by solvent treatment in conventional manner using boiling acetone as the solvent. The amount of asphalt removed amounted to 13% by weight of the original oil.

Percent wt. Vacuum residue 46.5 Gas oil 48.4 5.1

Propane precipitated asphalt Vacuum residue Bottoms product from the redistillation of a cat- The treatment in each case consisted in dilution of one part by weight of the fuel oil with five parts by Weight of methylene chloride, allowing the mixture to stand at ambient temperature (about 16-l8 C.) for half an hour and then filtering through a No. 4 BSS porosity sintered glass filter. The filtrate was distilled to recover the fuel oil and the last traces of methylene chloride removed by air blowing. The Wax remaining on the filter was dissolved off with hot benzene, the benzene evaporated and the residue weighed. The fuel oils had previously been cooled at the rate of about 1 C. per day from the blending temperature (about 49 C.) to the storage temperature. Fuel oil D was also treated after it had been rapidly cooled (about 6 C. per minute) as described in UK. patent specification 851,135.

The results of the treatment are given in the table. It will be seen that large cloud point reductions were obo tained by the treatment and the filtration quality of the oils was improved although only about 0.5% wt. of Wax based upon the total Weight of oil was removed. The wax content of the oils as determined by the method de scribed remained virtually unchanged by the treatment in most cases though the melting point of the wax decreased Wax precipitated from asolution of asphalt-free oil in methylene chloride at -32 C.

The BF cloud point test was a microscopic method which consists in cooling a small sample on a microscope slide and observing the temperature at which wax crystals become visible.

Table Fuel Oil A B C D Treatment Temperature, C Original 18 Original 16 Original 18 Original 18 18 oil oil oil oil Yield of oil, percent wt 99. 97.6 99. 2 99. 7 97. 0

Yield of extract (wax), percent wt O. 52 0. 34 0. 0.88

Inspection data on oil:

Specific gravity at 16 C./l6 C. 0.931 0. 932 0.923 0. 925 0. 924 0. 928 0.951 0. 952 0. 954 Kinematic viscosity at 38 C 52. 4 53. 8 24. 0 32. 7 42. 7 63. 7 62. 5 63.7 74. 7 Pour point (IP Upper), C. 12 23 -7 7 32 29 26 -26 -23 Asplialrene content, perccnt wt 2. 2 2. 8 2. 6 3. 2 3. 8 4. 4 4. 7 Wax Content, percent wt 7 7 7 7 6 6 5 4 4 Wax melting point, C 64 44 57 44 57 43 63 41 39 BP Filtration test:

100 mesh 2. 5 3. 8 2. 4 2 4 2. 3 200 mesh 2. 3 Blocked 2. 3 2. 4 2. 4 Hagemann & Hamm 2. 0 2. 1 2. 0 2. 0 Cloud Point, C 47 9 39 19 27 3 7 2 No'rE.-The figures in the last column apply to a sample of oil D that had been rapidly cooled (about 6 0. per minute) from the blending temperature (49 C) owing to the fact that high melting point wax had been removed. The carbon number distribution obtained by the mass spectrometer on the Wax removed from fuel oil A showed a range of C to C with a peak around C The Wax separated from the fuel oils was found to contain a small amount (ca. 6% wt.) of asphaltic material. The wax remaining in the oil contained substantially no material of C or higher.

The methods of test used were as follows:

Specific gravity, IP' 59 Kinematic viscosity, IP 71 Redwood I viscosity, IP 70 Cloud point, BP 92/53 Pour point, IP 15 Asphaltene content, IP 143 Wax content, BP 237/55 Wax melting point, IP 55 IP=British Institute of Petroleum The BF filtration test consisted in passing the sample of oil through filters composed of 100 mesh BSS gauze, 200 mesh BSS gauze, and 10 layers of 200 mesh gauze, (Hagernann and Hammerich) under a carefully controlled pressure at the laboratory ambient temperature and obtaining a ratio for the times of flow through two vessels of equal capacity. A perfectly filterable oil would give ratios of 2.4 using the 100 mesh filter, 2.2 using the 200 mesh filter and 2.0 using the Hagemann and Hammerich filter. The samples of oil were heated to 82 C. and then slow-cooled from 49 C. to 7 C. at about 1 C. per day prior to the test.

The BF wax content test consisted in measuring the We claim:

1. A method of treating a petroleum fuel oil having a viscosity in the range of 5 to centistokes at 38 C. and a specific gravity at 16 C. in the range of 0.90 to 0.96 to remove therefrom substantially all of the wax molecules having carbon numbers greater than 35, while removing substantially none of the Wax molecules having carbon numbers of less than 30, which consists essen tially of adding methylene. chloride to the Wax-containing fuel oil at a temperature within the range of about 2 to 27 C. to form a solution with all of the constituents of the fuel oil except the wax molecules having carbon numbers greater than 35, and passing the formed solution of methylene chloride and fuel oil and the undissolved wax through a filter at ambient temperature to remove substantially all of the wax molecules having carbon numbers greater than 35 as a precipitate on the filter.

2. A method as in claim 1 wherein from about 5% to 25% of the initial wax content of the oil is removed.

3. A method as in claim 1 wherein the fuel oil is a residual fuel oil.

References Cited by the Examiner. V

UNITED STATES PATENTS 2,172,320 9/1939 Jones 20833 3,093,752 6/1963 Benedict 20835 DELBERT E. GANTZ, Primary Examiner.

DANIEL E. WYMAN, Examiner.

H. LEVINE, Assistant Examiner, 

1. A METHOD OF TREATING A PETROLEUM FUEL OIL HAVING A VISCOSITY IN THE RANGE OF 5 TO 100 CENTISTOKES AT 38*C. AND A SPECIFIC GRAVITY AT 16*C. IN THE RANGE OF 0.90 TO 0.96 TO REMOVE THEREFROM SUBSTANTIALLY ALL OF THE WAX MOLECULES HAVING CARBON NUMBERS GREATER THAN 35, WHILE REMOVING SUBSTANTIALLY NONE OF THE WAX MOLECULES HAVING CARBON NUMBERS OF LESS THAN 30, WHICH CONSISTS ESSENTIALLY OF ADDING METHYLENE CHLORIDE TO THE WAX-CONTAINING FUEL OIL AT A TEMPERATURE WITHIN THE RANGE OF ABOUT 2 TO 27*C. TO FORM A SOLUTION WITH ALL OF THE CONSTITUENTS OF THE FUEL OIL EXCEPT THE WAX MOLECULES HAVING CARBON NUMBERS GREATER THAN 35, AND PASSING THE FORMED SOLUTION OF METHYLENE CHLORIDE AND FUEL OIL AND THE UNDISSOLVED WAX THROUGH A FILTER AT AMBIENT TEMPERATURE TO REMOVE SUBSTANTIALLY ALL OF THE WAX MOLECULES HAVING CARBON NUMBERS GREATER THAN 35 AS A PRECIPITATE ON THE FILTER. 